In a ducted fan, such as is commonly used in an aero engine for example, a fan is disposed coaxially within a duct and is driven to rotate within the duct to direct air rearwardly through the duct.
For efficiency and stability of the fan blades, the gaps between the tips of the blades and the inner casing of the duct, within which the fan rotates, must be kept to a minimum so as to minimise the leakage of air around the tips of the blades.
For a conventional fan blade, in order to eliminate damage to the blade and ultimately maximise its longevity, there would be a sizeable clearance gap between the blade tip and the fan case, to ensure that even under heavy manoeuvre loading there would be no contact with the inner surface of the fan case. However, an increased clearance gives a large specific fuel consumption penalty due to aerodynamic losses at the tip of the fan blade. Tip leakage is caused by the working fluid (i.e. air) tending to migrate from the concave or “pressure” surface to the convex or “suction” surface of the aerofoil through the gap between the tip of the blade and the stationary casing. The leakage occurs because of a pressure differential, and leakage causes flow disturbances over a large proportion of the aerofoil surface. These flow disturbances across the blade surface also cause a reduction in efficiency of the blade which results in a reduction of performance of the fan system.
Such flow disturbances also contribute to noise, and increasingly noise legislation places severe constraints upon engine design, with a key component of engine noise being that generated by the fan itself.
For previously considered fan systems, a so called “tip rubbing” solution is used in which the duct casing is provided with a lining comprising a sacrificial abradable layer which in certain operating conditions is designed to be cut or rubbed away by the blade tips as the fan blade passes the surface of the fan casing. The liner is sometimes referred to as a fan track liner (FTL). This approach helps to minimise the gap between the static casing and the rotating blade, thereby reducing tip leakage. However, this approach can only provide optimal sealing at maximum speed, when the blades are at maximum elongation, and not at cruise speeds where in a long haul flight the engine will spend most of its time and use most of its fuel.
For aerofoils such as wings, it is known that winglets improve the aerodynamic performance of the wing and hence of the aircraft. A winglet is typically a relatively small wing surface disposed on the tip of the main wing at right angles to the spanwise direction of the wing. The use of winglets at the tip of the wing reduces wing vortices and also noise, and lengthens the effective wing.
Previously considered winglet systems include those employed in turbine blades. Adaptation of such winglet designs for use with fan blades would require major changes to the fan architecture. More mass would be located at the tip of the fan blade, again requiring considerable redesign of the blade itself. Although turbine systems do rotate at similar rotational speeds to fans they are much smaller and much lighter. The forces on a similar system for a fan blade would be very large due to the extra mass and much greater radius of the fan blade as compared with a turbine blade. For a fan blade of composite material these problems would occur to a greater degree than with conventional metallic blades. The addition of further components to the blade would also affect stress concentrations and lead to potential initiation sites for damage and delamination within the composite fan blade.
Winglet systems have also be previously considered for cooling fans. However, these are low speed, low mass systems usually of plastic material and typically with a large clearance between the fan and its casing. An injection moulded plastic fan would be impractical and unworkable for a large turbofan engine since such blades have low strength, low integrity, low fatigue life and poor impact resistance. They also suffer unduly from “creep”—i.e. elongation under centrifugal forces.
Previously considered winglet systems, if used in fan systems, would also likely be uncontained by current so called “containment systems”. These are typically structures which are employed in the fan casing to contain fragments of detached fan which may, in very exceptional circumstances, be released for example when a fan blade is struck by an object, such as a bird, leading to a so called fan blade off (FBO) event. The width of the tip, increased by the presence of a winglet, would reduce the pressure energy of the fan blade fragment significantly, which could then prevent the fan blade fragment from penetrating the liner of the casing, which is necessary for it to be retained and contained by the fan case.